1. Field of the Invention
The present invention relates to a boost switching power supply, and more specifically, to a boost switching power supply that can produce an output voltage that is higher than its input voltage.
2. Description of the Related Art
As a boost switching power supply that can produce an output voltage that is higher than its input voltage, there is known, for example, the one represented by the circuit diagram in FIG. 1 of Japanese Patent No. 2,512,670. This circuit consists of a first closed circuit composed of an input power supply, a magnetic coil and a switching means, and a second closed circuit having an input power supply, a diode and a smoothing capacitor. When the switching means is ON, the magnetic coil is excited by electric current flowing to the first closed circuit composed of the input power supply, the magnetic coil and the switching means. When the switching means is OFF, current flows to the second closed circuit composed of the input power supply, the diode and the smoothing capacitor, exciting energy accumulated in the magnetic coil is discharged and the smoothing capacitor is charged. Thus, an output voltage higher than the voltage of the input power supply appears between the electrodes of the smoothing capacitor serving as the output terminals.
Generally in this boost switching power supply, the switching frequency is fixed and the duty factor (defined as the time ratio of switching means ON time to the switching period) is controlled so as to stabilize the output voltage. Because the operation of the device is explained in detail in the publication of the above patent, further explanation for the device will be omitted.
This boost switching power supply has a disadvantage of experiencing a huge recovery current. During the OFF period of the switching device, current equal to the input current flows in the diode. In this condition, however, when the switching means turns ON, the current flow in the diode falls rapidly and recovery current flows in the reverse direction for a short time owing to the characteristic of the diode. This recovery electric current increases in proportion to the magnitude of the current flow in the diode immediately before the switching means turns ON and the rate of decrease of the current in the diode owing to the switching means turning ON. In this boost switching power supply, the only factors operating to suppress the rate of decrease of the current in the diode is are the parasitic impedance contained in the circuit and the resistance arising when the switching device changes from OFF to ON. The rate of decrease of the current flow in the diode is therefore extremely fast and, consequently, a very large recovery current arises. The recovery current decreases the efficiency of the boost switching power supply because it increases the switching loss. Moreover, the switching loss increases in proportion to the drive frequency because it occurs at every switching of the switching means. For this reason, an attempt to miniaturize the boost switching device by increasing the drive frequency leads to increased switching loss. Furthermore, because the pulse wave of the recovery current is extremely steep, a large noise occurs when the huge recovery current starts to flow in the circuit. A sophisticated noise-suppression measure is therefore necessary.
Various methods are available for solving the above mentioned problem of the boost switching power supply disclosed in Japanese Patent No. 2,512,670. The boost switching power supply taught in WO 98/35432 employs one such method.
In the boost switching power supply disclosed in WO 98/35432, the second magnetic coil is inserted into the first closed circuit consisting of the input power supply, the magnetic coil (the first magnetic coil) and the switching means. By intervention of the second magnetic coil, the rate of increase of the current flowing to the switching means when the switching means changes from ON to OFF is restricted. Consequently, the rate of decrease of the current flowing in the diode when the switching device changes from ON to OFF becomes slow and the recovery current can be restricted effectively.
However, in the boost switching power supply disclosed in WO 98/35432, since the second magnetic coil is inserted in the first closed circuit for exciting the first magnetic coil serving as a main coil, current flows constantly in the second magnetic coil for exciting the first magnetic coil. Thus, another problem arises in that the efficiency of the boost switching power supply is degraded. Moreover, because huge current flows in the second magnetic coil, the size of the entire device needs to be enlarged, so that device miniaturization is hampered.
Another method for solving the above-mentioned problem of the boost switching power supply taught in Japanese Patent No. 2,512,670 is that used in the boost switching power supply disclosed in U.S. Pat. No. 5,418,704.
In the boost switching power supply disclosed in U.S. Pat. No. 5,418,704, the second magnetic coil and the second switching means are connected in series and this series connection is connected in parallel with the switching means (the first switching means) to constitute a third closed circuit composed of a smoothing capacitor, second magnetic coil and second switching means. Immediately before the change of the first switching means from OFF to ON, the second switching means is changed from OFF to ON. As pointed out above, in the boost switching power supply taught in the above-mentioned publication, because the second switching means, serving as an auxiliary switch, is changed from OFF to ON immediately before the first switching means, serving as a main switch, changes from OFF to ON, the substantially no recovery current flows into the first switching means. The switching loss occurring in the first switching means is therefore reduced. Furthermore, because the third closed circuit contains the second magnetic coil, the switching loss occurring in the second switching device during the transition from OFF to ON is also reduced.
However, in the boost switching power supply disclosed in U.S. Pat. No. 5,418,704, the transition from ON to OFF of the second switching means is hard switching in which the turn-off is effected while current is flowing. For this reason, turn-off loss occurs in the second switching means.
As indicated above, although various methods have been attempted to lower the switching loss owing to the recovery current, the boost switching power supply by these methods have other problems as mentioned above. For these reasons, a boost switching power supply with low switching loss is desired.